Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 18 Fluidized bed reactor for immobilized lipase–catalyzed interesterification.
A. pH
Lipases are only catalytically active at certain pHs, depending on their origin and
the ionization state of residues in their active sites. While lipases contain basic,
neutral, and acidic residues, the residues in the catalytic site are only active in one
particular ionization state. The pH optima for most lipases lies between 7 and 9,
although lipases can be active over a wide range of acid and alkaline pHs, from
about pH 4 to pH 10 (50,99). For example, the optimum pH for lipase from Pseudomonas
species is around 8.5, whereas fungal lipases from Aspergillus niger and
Rhizopus delemar are acidic lipases (100). The effect of immobilization on the pH
optimum of lipases is dependent on the partitioning of protons between the bulk
phase and the microenvironment around the support and the restriction of proton
diffusion by the support. If the lipase is immobilized on a polyanion matrix, the
concentration of protons in the immediate vicinity of the support will be higher than
in the bulk phase, thereby reducing the pH around the enzyme in comparison with
the pH of the bulk phase. Since there is a difference in the perceived pH of the
solution as measured by the pH of the bulk phase, the lipase would exhibit a shift
in pH optimum toward a more basic pH. For instance, for a free lipase that has a
pH optimum of 8.0, when immobilized on a polyanionic matrix, with the bulk solution
at pH 8.0, the pH in the immediate vicinity of the lipase might be only 7.0.
Therefore, while the reaction pH is 8.0, the lipase is operating at pH 7.0, which is
below its optimum. The pH of the bulk solution would have to be increased to pH
9.0 to get the pH around the lipase to its optimum of 8.0. This phenomenon is only
seen in solutions with ionized support and low ionic strength systems (101). If protons
are produced in the course of interesterification, the hydrogen ion concentration
in the Nernst layer can be higher than in the bulk phase, thereby decreasing the pH
in the vicinity of the lipase. Running an interesterification reaction with lipases at a
pH well removed from the optimum can lead to rapid inactivation of the enzyme.
B. Temperature
In general, increasing the temperature increases the rate of interesterification, but
very high temperatures can reduce the reaction rates due to irreversible denaturation
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.